This invention relates to ferromagnetic elements possessing temperature regulation when electrically heated.
It is old and well known to regulate the temperature of a ferromagnetic element by passing a radio frequency current through it. The current heats the element to its effective Curie temperature where due to a change in permeability of the element, the power drawn by the element declines, and therefore the device will hold its temperature constant. In some prior art devices, the skin depth within which the radio frequency travels increases (and the resistance of the element to the radio frequency current) and the permeability of the element declines as the temperature of the element approaches its effective Curie temperature. The "effective" Curie is the temperature at which the device regulates its temperature and is 50.degree.-100.degree. C. below the actual published Curie temperature. Hereafter, when reference is made to the Curie temperature it should be understood that the reference is to the effective Curie, unless otherwise stated. The current may be fed through the ferromagnetic element directly, as by electrical conductors connected between the element and a source of current, or by induction. However, the known prior art employing a pure ferromagnetic element has the drawback that it will not hold the temperature constant over a wide range of cooling loads.
An improvement in the aforesaid temperature regulation method is shown and described in U.S. Pat. No. 4,256,945, issued Mar. 17, 1981, to Philip S. Carter and John F. Krumme, entitled Alternating Current Electrically Resistive Heating Element Having Intrinsic Temperature Control. This patent teaches that the temperature regulation may be improved if the ferromagnetic element surrounds a copper substrate. Below the effective Curie the current is driven into the ferromagnetic surface layer by strong skin effect forces. When the temperature rises to the effective Curie temperature the skin effect is not strong due to the change in permeability of the ferromagnetic material, and at least some of the current retreats into the copper. This results in a sharp drop in power; since the current is held constant throughout the process. Hence, it is possible to design such a device that holds its temperature constant over a wider range of thermal cooling loads than was the case with the ferromagnetic element.
The Carter-Krumme patent in col. 7 states the effectiveness of the device in terms of R.sub.max where R.sub.max is the resistance of the device below Curie and R.sub.min is the resistance of the device above Curie.
The Carter-Krumme patent teaches that the preferred frequency range is 8 to 20 MHz.